The invention relates to a drive for a piston in a container containing an injectable product.
Portable injection and/or infusion devices are used to administer for example medicines in fluid form, in particular in liquid form, for example insulin. The medicine fluid is displaced from a fluid container by means of a piston and administered in precise doses.
Drive devices for the piston of the type which are preferred for the purposes of the present invention are known from WO 98/47552. These are telescopic drives, in which a first shifting stage shiftably mounted in or on a base part advances the piston in an advancing direction in the container when it is itself advanced, displacing the product from the container. In addition to the first shifting stage, at least a second shifting stage is provided which can be shifted in the advancing direction relative to the base part and which slaves the first shifting stage in the advancing direction when it is shifted. The shifting stages are shifted relative to the base part and to each other by spindle drives. By using a number of spindle drives, a long shifting path can be set precisely. However, thread pairings, such as those formed by spindle drives, exhibit axial play. The magnitude of the play is determined by the thread tolerances.